1. Field of the Invention
The invention relates to an imaging apparatus, a signal processing circuit, a signal processing apparatus, a signal processing method, and a computer program product. In particular, the invention relates to a technology applied to any of them in which information, at a level not determined by quantization, of an original signal is reproduced from a video signal output as a digital signal.
2. Description of the Related Art
When converting an analog signal to a digital signal, the amount of information in the digital signal can be determined by a sampling frequency and the number of quantization bits (quantization bit rate). The sampling frequency determines the maximum frequency that can be represented by the Nyquist's theorem, while the quantization bit rate determines the accuracy in the direction of amplitude. In other words, the quantization bit rate determines the minimum variation of the digital signal. If the minimum variation determined by the quantization bit rate is too large with respect to the signal expressed, human beings will perceive so-called quantization distortion.
Therefore, for preventing the quantization distortion from being obvious, the dynamic range of analog signal input to an analog-to-digital converter (hereinafter, referred to as an A/D converter) is adjusted in advance to the dynamic range of the A/D converter. However, if such processing is carried out, information, at a level not determined by quantization in the A/D convertor, of an original signal may disappear.
FIG. 1A illustrates an example of the waveform of an analog signal input into an A/D converter. FIG. 1B illustrates an example of the waveform of a signal when the dynamic range of the analog signal input into the A/D converter is adjusted to the dynamic range of the A/D converter. In this example, the A/D converter has a quantization bit rate of 8 bits. The example shown in FIG. 1B illustrates that the information corresponding to a high accuracy portion over the range of 8 bits as indicated by the arrow in the figure disappears when passing through the A/D converter.
In addition, the digital-signal processing in a video camera or the like may lead to a phenomenon in which the minimum variation of a digital signal becomes too large with respect to an expressed signal. For example, if a steep gain adjustment is performed when shooting in a dark place or the like, the least significant bit data of the original signal will be shifted to high-order bits. As a result, there may be a disadvantage in that an expressed video may have impression of insufficient gray scale. A similar disadvantage may occur when carrying out a gamma correction for correcting the gamma characteristics of signals of three primary colors to fit to the gamma characteristics of a monitor.
While having these disadvantages, an effective measure for smoothly expressing the gray scale of an output signal is to increase the quantization bit rate. However, the performance of the A/D converter is typically limited. Thus, optionally increasing the quantization bit rate is difficult. Besides, in view of the production costs, there is a case where an A/D converter with a high quantization bit rate may not be used.
Without an increase in quantization bit rate in the A/D converter, for example, there is a method of expanding the number of bits in the direction of the least significant bit of a signal output from the A/D converter and random noise is incorporated into the expanded portion. Another method, which has been employed in the art, is to reproduce a high precision component, at a level not determined by quantization in the A/D convertor, of an original signal from a digital signal after A/D conversion while retaining the information of the original signal input into the A/D converter.
FIG. 2 represents an example of configuration of a circuit for the reproduction of a high precision component, at a level not determined by quantization, of an original signal. The circuit that reproduces the high precision component at a level not determined by quantization is hereinafter referred to as a quantization-accuracy reproducing circuit. For example, this circuit can be applied to a video camera or the like. In order to correct an insufficient gray scale occurred as a result of raising the least significant bit in an analog-to-digital converted original signal or in a digital signal having a significantly adjusted gain, the circuit generates a high precision component (“m” bits) added to the lowest bit from an original signal (“n” bits) to obtain a quantization bit rate of “n+m” bits.
The quantization-accuracy reproducing circuit includes, for example, a low-pass filter 201, a high-precision component separating part 202, and a high-precision component adding part 203. The low-pass filter 201 generates a digital signal with a bit length of “n′+m”, which contains a signal component (high precision component) at a level not determined by quantization. FIGS. 3A to 3D illustrate an example of output of the low-pass filter 201 in the case of m=1. In the example shown in FIGS. 3A to 3D, a digital signal obtained by digitization of an original signal has a signal length “n” of 8 bits.
The low-pass filter 201 calculates an average value of two digital signals to output a signal containing bit information at a level not determined by quantization of the original signal. FIGS. 3A-1 and 3A-2 illustrate two digital signals to be processed, respectively. These signals are 8-bit digital signals and then added together to form a 9-bit output signal as shown in FIG. 3B. In other words, these signals pass through the low-pass filter 201, thereby generating a signal with a bit length of 9 bits (i.e., “n(8)+m(1)” bits=9 bits).
A broken line extending in the vertical direction in FIGS. 3A to 3D indicates the decimal point. The signal shown in FIG. 3B is divided to adjust digits as shown in FIG. 3C. Then, the calculated resulting part (1 bit) after the decimal point as shown in FIG. 3C is a high precision component. As shown in FIGS. 3C and 3D, the high-precision component separating part 202 shown in FIG. 2 carries out the process of separating “m” bits of high-precision component from a signal output from the low-pass filer 201. Subsequently, the high-precision component adding part 203 adds the m bits separated by the high-precision component separating part 202 to the least bit of the “n”-bit digital signal (original signal).
In other words, an “n”-bit original signal as shown in FIG. 4A has lost a component at a level not determined by quantization as indicated by the broken line. However, by passing through the low-pass filter 201, the signal containing the component at a level not determined by quantization can be generated as shown in FIG. 4B. Subsequently, an “m”-bit high-precision component is removed from the above signal by the high-precision component separating part 202. Furthermore, as shown in FIG. 4D, a signal is generated such that the “m”-bit signal separated in FIG. 4C is added to the original “n-bit signal of FIG. 4A by the high-precision component adding part 203. Such a process leads to the reproduction of the high precision component without an increase in quantization bit rate in an A/D converting part.
However, according to the above-described method, reproduction of the high precision component may receive influence of high-frequency components such as edge and noise. In addition, improvement in effects of the reproduction of the quantization accuracy may lead to deterioration of the image quality. Accordingly, a method for detecting a high precision component and controlling the reproduction of the high precision component depending on the amount of the detected high precision component has been employed in the art.
Japanese Unexamined Patent Application Publication No. 2006-222479 discloses that a high precision component is detected and the effects of reproducing a high precision component are then controlled on the basis of the amount of the detected high precision component.